The comments by Sir Ian Blair, the Met's Commissioner, followed the shooting of Jean Charles de Menezes, 27, an innocent Brazilian electrician, who was killed by police on Friday at Stockwell Tube station, south London.
The death has provoked strong criticism of the police by the man's family, who described the Met as "stupid and incompetent".
Sir Ian said: "Somebody else could be shot. But everything is done to make it right. This is a terrifying set of circumstances for individuals to make decisions."
The country's most senior police chief defended the actions of his officers, saying: "What we have got to recognise is people are taking incredibly difficult fast-time decisions in life-threatening situations.
"It wasn't just a random event and what's most important to recognise is that it's still happening out there. There are still officers out there having to make those calls as we speak."
Since the 11 September 2001 terrorist attack on the United States the British police has adopted a "shoot-to-kill" policy when dealing with suspected suicide bombers.
Previously, firearms officers were trained to shoot someone who "posed a threat to life" in the body, usually twice, to disable and overwhelm them. But with the threat of suicide attacks, that was altered to allow officers to shoot a suspect - who was thought to be carrying explosives - in the head. The firearms officers are told to shoot the suspect several times so that they cannot activate a bomb, and not to shoot the body in case the bullets detonate any explosives.
The procedures would remain in place, insisted Sir Ian. "They have to be that because there is no point in shooting at someone's chest because that is where the bomb is likely to be.
"There is no point in shooting anywhere else if they fall down and detonate it. It is drawn from experience from other countries, including Sri Lanka. The only way to deal with this is to shoot to the head."
The policy had been "reviewed and reviewed" for many months and was a national one, not just for London, he said.
Jack Straw, the Foreign Secretary, also defended the policy. He said it was essential police were able to deal effectively with the threat of a suicide attack.
An investigation has been launched into Friday's killing.Reuse content